News | March 7, 2018

ARRL Repurposes AM Broadcast Transmitter For Ham Radio Use

Thanks to a joint effort by ARRL and the Vintage Radio and Communications Museum of Connecticut (VRCMCT), a classic Gates BC-1T AM broadcast transmitter will enjoy a second life on the Amateur Radio bands for occasional use under W1AW or under the ARRL Headquarters Operators Club call sign, W1INF. Spearheaded by broadcast engineer Dan Thomas, NC1J, VRCMCT volunteers restored the1-kW transmitter to operating condition, after obtaining it from the National Capital Radio and Television Museum in Bowie, Maryland. The VRCMCT will retain ownership of the transmitter, while the League houses, and maintains it on loan. The transmitter will be located in the ARRL Lab, and Assistant Lab Manager Bob Allison, WB1GCM, said the transmitter could be on the air as W1AW during such operating events as the AM Rally and the Heavy Metal Rally.

ARRL turned to AM guru and veteran broadcast engineer Tim “Timtron” Smith, WA1HLR, of Skowhegan, Maine, to handle shifting the BC-1T from 1340 kHz to the ham bands. Timtron not only has been an AM mainstay on 75 and 40 meters over the years, he’s engineered all manner of AM, FM, and HF broadcast transmitters in his extensive career. This combination of familiarity and experience made him a logical choice to handle the conversion to amateur use of the Gates BC-1T, which once transmitted country music from KPGE in Page, Arizona.

While a shift from the higher end of the Standard Broadcast Band to 160 meters alone might seem rudimentary, various stipulations added a level of complexity. First, the transmitter had to be modified as little as possible, retaining original components. That ruled out completely redesigning the circuitry. The 833 final amplifier tubes, better suited for broadcast-band use, would be retained as would the inductance-heavy tuning circuits. Another requirement — this one set by Smith — ambitiously called for the transmitter to function on 75 as well as on 160 meters.

The plan was to accomplish the conversion in two phases, with the first to be completed in a few days and include basic crystal-controlled functionality on 160 and 75. The second phase will include adding remote control, relay band switching, and external RF drive for frequency agility, to be completed later.

Each RF stage was converted, starting with the Colpitts oscillator — which offered two octal tube sockets to hold broadcast crystals, and a selector switch. As most veteran radio amateurs may recall from their Novice license days, an octal socket will accommodate FT-243-case crystals; in this case, only minor rewiring was needed. More complicated was changing out feedback and loading capacitors in the oscillator stage, along with the buffer tank circuit. The driver tank circuit was next. Removing one-half of the windings on the multiple tank, changing some connections, shortening long leads on RF bypass capacitors, and modifying the neutralization circuit were necessary.

The output tuning circuit proved to be the easiest to convert; parallel capacitors that enabled broadcast-band operation were rewired in series to resonate on the amateur bands. A spare inductor, not required for higher frequencies, was repurposed in place as a dc safety shunt. The modulator just needed only minor changes. All was documented.

Initial tests conducted at 250 W on February 22 demonstrated the success of the modifications and marked completion of the first step toward a new lease on life for the BC-1T as ARRL’s flagship AM amateur band transmitter. “It took many volunteers and their resources to make this project come together,” Allison said. Eventually, visitors to ARRL Headquarters will be able to see the transmitter on the air and possibly use it, by advance request. Allison calls the BC-1T “The Ambassador.”

“It’s an ambassador for the AM mode, reaching out a friendly hand to radio amateurs old and new,” he said. — Thanks to Clark Burgard, N1BCG, and Bob Allison, WB1GCM.

Source: ARRL